Such a steering system is known, for example, from EP 2 186 710 A1.
The use of two steering circuits has the advantage of a greater reliability. The risk of a dangerous situation during the operation of a vehicle equipped with such a steering system is reduced. When a vehicle equipped with such a steering system is driven, the driver actuates a steering member, for example, a steering wheel, which is connected to the control valves. Depending on the direction of the steering movement the control valves open flow paths from the respective high pressure ports to one of the working ports and another flow path from the other working port of each steering circuit to the respective low pressure port. In each flow path a number of throttling means in form of orifices or bleeds are arranged. Before entering the respective control valve the fluid from the high pressure port is guided through the measuring motor. The measuring motor acts on the control valve in a sense to close the above mentioned flow paths when an amount of hydraulic fluid corresponding to the intended steering angle of the steering member has been delivered or metered to the respective working port.
In the steering system of the prior art the control valves each comprise a housing having a bore in which a spool and a sleeve are positioned. The spool and the sleeve can be rotated relative to each other to open the above mentioned flow paths and to close other flow paths, if appropriate.
Such a steering system has the drawback that a change from one steering direction to the opposite steering direction shows a certain gap or a dead zone. In other words, when the steering direction is changed a certain time passes before the steering motors can react to the new steering command. This makes in some cases steering difficult and uncomfortable.